76 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
of the neuro-muscular sheet most remote from the 
ganglion be gently brushed with a camel’s hair 
brush—z.e. too gently to start a responsive contrac- 
tion-wave—the ganglion at the other end will 
shortly afterwards discharge, as shown by its start- 
ing a contraction-wave at its own end of the 
parallelogram, b; thus proving that the stimulus 
caused by brushing the tissue at the other end, a, 
must have been conducted all the way along the 
parallelogram to the terminal ganglion, b, so causing 
the terminal ganglion to discharge by reflex action. 
Indeed, in many cases, the passage of this nervous 
wave of stimulation admits of being seen. For the 
numberless tentacles which fringe the margin of 
Aurelia are more highly excitable than is the 
general contractile tissue of the bell; so that on 
brushing the end a of the parallelogram remote 
from the ganglion, the tentacles at this end respond 
to the stimulus by a contraction, then those next in 
the series do the same, and so on—a wave of con- 
traction being thus set up in the tentacular fringe, 
the passage of which is determined by the passage 
of the nervous wave of stimulation in the super- 
jacent nervous network. This tentacular wave is 
in the illustration represented as having traversed 
nearly half the whole distance to the terminal gang- 
lion, and when it reaches that ganglion it will cause 
it to discharge by reflex action, so giving rise toa 
visible wave of muscular contraction passing in the 
direction b a, opposite to that which the nervous or 
tentacular wave had previously pursued. Now this 
tentacular wave, being an optical expression of a pas- 
